The invention is particularly useful for calibration of a portable robot. With a portable robot is meant a robot which is mobile of its own or is movable by other means, such as a truck. A work area is a place at which the robot shall carry out work, for example, a robot cell. For example, the work area includes work objects and/or workstations on which the robot is to carry out work.
A robot includes a base portion and a plurality of parts movable relative the base portion. A base coordinate system is defined in a fixed relation to the base portion. During calibration of the base coordinate system, the position and orientation of the base coordinate system, and accordingly the position and orientation of the base portion, is determined in relation to the work area.
A common situation, especially in smaller enterprises, is that robot automation is not affordable because robots can not move between different machines as flexible as a human worker. Sometimes it is possible to mount the robot hanging on linear guide ways or standing on floor-mounted guide ways, but this kind of solution is expensive and can be difficult to install in the workshop. What is instead needed in this type of situations is a low cost portable robot easy to move between different robot cells and where time needed to get the production running after the movement of the robot is very short. Another application of such a robot solution is in cases with large work objects, where the robot must be moved between different parts of the work objects. A problem with moving a robot between different work areas is that the position and orientation of the robot with respect to the work area must be determined with a high accuracy each time the robot has been moved to a work area. Concepts presented so far to solve this problem are too complicated with special docking mechanisms, docking sensors and even navigation systems. There is also the cost problem when too complex platforms are used, including safety sensors, high precision docking mechanisms, docking sensors etc. The most important reason why portable robots have not yet been introduced is that the concepts presented so far do not solve the problem of how to reduce the time between the docking of the robot and the execution of the robot programs.
A basic problem in robot installations and commissioning of robot cells is the identification of the relation of the robot to a work piece to be processed by the robot. The robot is supposed to perform work on the work piece and the work piece is located in the working range of the robot. The reason for the work piece identification is to define a local coordinate system in the work piece that can be used for programming the robot to perform work on the work piece. WO2009132703 discloses a method for calibration of a local coordinate system relative a robot coordinate system. A sphere is mounted on the robot and at least 3 mating targets on the local object. Each mating target includes two or three nonparallel plane surfaces arranged to receive the sphere so that the sphere is in contact with the surfaces. By moving the robot to mate the sphere with the three targets in a sequence it is possible to calculate the local coordinate system in relation to the robot coordinate system. Force control is exemplified as a possibility for making the mating. This method is difficult to use for the calibration of the base coordinate system of a robot on a movable platform for the following reasons:                When the robot has got a new pose it is difficult to adjust the robot calibration program to mate the sphere with the three targets since these are distributed in the robot cell.        After a movement of the robot there is a large risk that the robot will not be able to reach all the three targets.        Since a fairly large distance is needed between the targets the robot inaccuracy will always degrade the calibration.        At each mating the robot tool can have any orientation, which further reduces the accuracy of the method when used for base coordinate system calibration.        Using lead through for making the mating, which is the most intuitive method, it will be very difficult and sometimes impossible to move the sphere to the three targets.        
For portable robots, the method in WO2009132703 is very difficult to use and is actually not developed for robot base coordinate system calibration but for the calibration of work objects and fixtures.